Akari Light Up

Overview

Akari, also known as Light Up, is about placing lamps so their beams cover every open square. A bulb illuminates its own cell and continues horizontally and vertically until a black wall stops the light. Two bulbs may not shine directly on each other. Numbered walls add local constraints: the number counts bulbs immediately above, below, left and right of that wall, not distant bulbs in the same line. Unnumbered walls simply block light. The whole board must be illuminated while satisfying both kinds of rule. The locally compiled Light Up engine provides new generated boards and different settings. There is no action timer, so you can pause to reason about a dark corridor before committing another bulb. Small boards teach the vocabulary of forced walls; larger ones turn those patterns into long chains of deductions.

How to Play

Click a white square to place a bulb, then click it again to remove it. Use a secondary click to mark a square where a bulb should not go. On a phone, the Secondary toggle provides that marking action without requiring a long press. Observe how each bulb lights its row and column up to the nearest walls. A completed board has no dark white squares, no bulbs facing each other, and the correct adjacent bulb count for every numbered wall. Undo reverses your latest choice. Accurate marking matters as much as accurate bulbs, because forbidden-square notes are what make later deductions visible.

Tips & Strategy

Mark the neighbours of a zero wall as unavailable immediately. A four forces bulbs on all four sides, while a three beside the outer edge may also have every possible position fixed. Once a bulb lights an unobstructed corridor, additional bulbs anywhere along that corridor are forbidden. Look for an unlit square with only one remaining place that could illuminate it; that deduction can be stronger than the nearest numbered clue. Remember that a lit cell may still be forbidden as a bulb location, and that meeting all wall numbers is insufficient if a remote corner remains dark.

Wall Numbers and Forced Squares

The wall digits form a ladder of standard openings. A 0 forbids every orthogonally adjacent square, so mark those cells before doing anything else. A 4 is the most generous clue on the board: bulbs go on all four sides at once. A 3 sitting on the outer edge has only three neighbours, so all three take bulbs, and a 2 tucked into a board corner forces both of its neighbours. After each forced bulb, two follow-up moves matter: light travels down its row and column, and every other square in the same unobstructed stretch becomes forbidden, since a second bulb there would face the first.

The other core technique is the only-source argument. Picture a white square near the bottom edge. The cell below it is marked forbidden by a 0 wall, its row is already lit by a distant bulb except for one short walled-off segment, and the only open cell sharing its column is the square itself. No wall number touches it, yet the bulb is forced: nowhere else can ever reach it. Scanning for squares whose potential light sources have all been eliminated solves boards that pure wall arithmetic cannot.

Mistakes That Darken the Board

The classic blunder is satisfying a wall number with a bulb that faces an existing bulb down a long corridor, which breaks two constraints at once. Players also forget that illumination and placement are different questions: a square can be brightly lit and still be an illegal bulb spot. Resist tidying up the numbered walls and calling the board done; sweep the whole grid for unlit cells, especially corners shielded by unnumbered walls. Finally, do not skip the marking step. Unmarked forbidden squares hide the only-source deductions that finish difficult boards.